The present invention relates to methods of cleaning the surfaces of articles, particularly microelectronic devices at one or more points during the manufacture of such devices. More particularly, the present invention relates to methods of cleaning surfaces of microelectronic devices using wet processing techniques in conjunction with the application of acoustic energy and/or in conjunction with an oxidizing pre-treatment. In preferred embodiments, cleaning liquids of the present invention are ultradilute solutions formed by dissolving a gas solute in a suitable solvent.
Since the early days of the microelectronic industry, the importance of minimizing contamination on microelectronic devices throughout the manufacturing process has been recognized. Contaminants include particles, photoresist residue, and/or the like whose presence can adversely impact the performance and function of microelectronic devices if not adequately removed. Accordingly, various cleaning treatments have been devised.
However, as the end product devices have become more and more miniaturized, a contaminant occupies an increased percentage of the available space for circuitry and other device elements. Hence, cleanliness of the materials has become far more critical, and cleanliness specifications have become increasingly more stringent. Unfortunately, these trends make cleaning much more challenging.
One traditional cleaning approach has involved the use of an aqueous solution of H2O2 and NH4OH to carry out cleaning in which the volume ratio of the peroxide to aqueous ammonia to water typically is 1:1:5. These relatively concentrated mixtures are commonly referred to as the Standard Cleaning Solution 1 (xe2x80x9cSC-1xe2x80x9d). The SC-1 process is believed to detach particles through surface etching. The use of megasonic energy in combination with the SC-1 chemistry is reported to improve particle detachment.
In basic solution, many common particle types will bear the same kind of surface charge as do silicon and silicon dioxide surfaces being cleaned, theoretically causing the particles and the surfaces to repel each other via electrostatic repulsion. However, SC-1 cleaning solutions tend to have very high ionic strength, causing the electrostatic repulsion forces to be negligible as a practical matter. Electrostatic repulsion forces therefore play only a minor role, if any, in connection with the SC-1 chemistry.
Although widely used, the SC-1 approach has drawbacks. These relatively concentrated solutions might clean effectively, but unfortunately they can also deposit metal contaminants onto the devices being cleaned. Of course, a cleaning method that deposits contaminants is counterproductive. These concentrated solutions can also unduly etch and damage the surfaces of the devices being cleaned. Device damage is also a result that is desirably avoided by a cleaning method. As still another drawback, the peroxide typically must incorporate stabilizers, and these can contaminate the surface being cleaned, which also is counterproductive especially when relatively high concentrations of peroxide are used.
Another cleaning approach is described in U.S. Pat. No. 5,656,097. This approach involves cleaning devices with aqueous solutions of ammonia and hydrogen peroxide in combination with the application of megasonic energy. In this approach, the dilute solutions are prepared by diluting more concentrated solutions of aqueous ammonia with water. The approach has drawbacks. Although dilute, these solutions can still unduly etch the devices being cleaned, metal contaminants can still be deposited, and the stabilizer for the peroxide is still a contaminant. Further, it is very difficult to prepare dilute solutions with good accuracy by diluting relatively small volumes of concentrated solutions with relatively large volumes of solvent. The inaccuracy can lead to differences in cleaning performance from device to device.
It can be seen, therefore, that improved cleaning methods that can satisfy these more stringent demands imposed by miniaturization are still needed.
The present invention provides an approach for cleaning articles, such as microelectronic devices at various stages of manufacture, that is extremely effective at removing particle contaminants and/or organic debris, e.g., photoresist remnants, from the device surfaces. In preferred embodiments, the approach accomplishes cleaning without depositing metal contaminants onto the surface of the devices and without undue etching or other damage of the surfaces. As used herein, the term xe2x80x9cmicroelectronic devicexe2x80x9d includes but is not limited to semiconductor wafers, integrated circuits, thin film heads, flat panel displays, microelectronic masks, and the like. The term shall also refer to partially completed devices as they are being manufactured.
The approach of the present invention is significant in at least two respects. First, preferred embodiments of the present invention achieve cleaning efficiencies of better than 99.9% with respect to particles having a size greater than about 0.16 microns. Second, in addition to high particle removal efficiency, it is also important to carry out cleaning operations without adversely affecting surface smoothness and without depositing additional contaminants, e.g., metal contaminants, onto the surface being cleaned. Preferred embodiments of the present invention do not adversely affect surface roughness in any significant way. In fact, preferred cleaning embodiments of the present invention have actually provided post-clean surfaces that are smoother than the same pre-cleaned surfaces Metal contamination of preferred embodiments is neutral, meaning that cleaning operations deposit substantially no, if any, metal contaminants onto the surfaces being treated.
Preferred embodiments of the invention carry out cleaning operations using the combination of acoustic energy and a cleaning liquid comprising an ultradilute concentration of a cleaning enhancement agent. Amazingly, the combined use of acoustic energy, particularly megasonic energy, and ultradilute cleaning reagents provides exceptional cleaning performance even though the amount of cleaning enhancement agent in the reagent is almost negligible as a practical matter. For example, reagents containing approximately 100 ppm gaseous anhydrous ammonia dissolved in filtered deionized water remove particles from substrates such as semiconductor wafers with very high efficiency.
In one aspect, the present invention provides a method of cleaning a surface of an article using ultradilute cleaning liquids in combination with acoustic energy. An ultradilute concentration of a cleaning enhancement substance, such as ammonia gas, is dissolved in a liquid solvent, such as filtered deionized water, to form a cleaning liquid. The cleaning liquid optionally may also include other ingredients, such as hydrogen peroxide, if desired, but such additives are not needed and may not even be desired to achieve excellent cleaning performance. The cleaning liquid is caused to contact the surface to be cleaned. Contact can occur by causing the liquid to flow past the surface, by spraying the liquid onto the surface, by submerging the surface in a body of the liquid, and/or the like. Preferably, acoustic energy is applied to the liquid during such contact.
In another aspect, the present invention provides a cleaning method in which a surface of an item to be cleaned is first contacted with a processing liquid comprising an oxidizing agent. A preferred processing liquid for this purpose is ozonated water, but solvents such as water containing other oxidants such as hydrogen peroxide could also be used if desired. Next, the surface is contacted with a cleaning liquid, preferably ultradilute aqueous ammonia. Acoustic energy is directed into the cleaning liquid during at least a portion of the time that contact with the cleaning liquid is occurring.
In another aspect, the present invention provides a cleaning method in which a substrate is positioned in a cleaning vessel with the surface to be cleaned being substantially vertical. A cleaning liquid comprising and ultradilute concentration of a cleaning enhancement substance, preferably ammonia, is then introduced into the vessel. As the vessel fills, the rising top surface of the cleaning liquid traverses the substrate surface. Acoustic energy is applied to the rising cleaning liquid.
In still another aspect, the present invention involves a method of cleaning a surface of an article in which an ultradilute concentration of a gaseous cleaning enhancement substance is dissolved in a liquid solvent to form a cleaning liquid. The cleaning liquid is caused to contact the substrate surface. While causing the cleaning liquid to contact the substrate surface, acoustic energy is applied to the cleaning liquid. After causing the cleaning liquid to contact the substrate surface, the substrate surface is rinsed and then dried. Preferably, drying occurs by contacting the substrate surface with a first process reagent comprising a carrier gas, preferably nitrogen, and a cleaning enhancement substance, preferably an ultradilute concentration of isopropyl alcohol. The substrate surface is also contacted with a drying reagent comprising a heated gas, preferably heated nitrogen.